The present invention disclosed herein relates to an optical device and a method of manufacturing the optical device, and more particularly, to a spot size converter appropriate for monolithic integration of an optical device, and a method of manufacturing the spot size converter.
Recent semiconductor optical devices are designed and manufactured based on the concept of a ridge waveguide (RWG) in order to improve high speed operation, simplicity of fabrication, and monolithic integration. For example, active devices such as laser diodes and optical amplifiers may have a shallow RWG structure by etching only an upper clad layer except for an active region. Passive devices such as optical waveguides, modulators, and phase controllers may have a deep RWG structure by performing an etching process to under an active region. The shallow RWG structure and the deep RWG structure can be efficiently and monolithically integrated by adjusting the width of a deep ridge waveguide.
In general, a guided mode of a semiconductor optical device having a shallow ridge waveguide may have an oval shape or a hat shape. Since such a guided mode has a far-field pattern (FFP) with a vertical full-width at half maximum (FWHM) different from a horizontal FWHM, when a semiconductor optical device having the guide mode is coupled to a typical optical fiber or a specific optical fiber, optical coupling loss is increased.
Since an FFP of a typical semiconductor optical device having a deep ridge waveguide has a FWHM greater than that of an optical device including an optical fiber or a silica waveguide, when the semiconductor optical device is coupled to the optical device including an optical fiber or a silica waveguide, optical coupling loss is also increased.
Thus, optical coupling efficiency between a ridge waveguide type semiconductor optical device and a device such as an optical fiber, a silica waveguide, or a polymer waveguide can be improved by varying an FWHM of an FFP of the ridge waveguide type semiconductor optical device. To this end, a semiconductor optical device may perform a spot size conversion through mode conversion and transfer by means of a core layer of a shallow ridge waveguide type laser diode, and a waveguide separately disposed under the core layer. However, even though the width of a shallow ridge waveguide is decreased, since variation in effective refractive index of a guided mode is not large, mode conversion efficiency is not high. To address this issue, the width of a ridge waveguide and the thickness of a clad layer disposed on an active layer may be gradually decreased to decrease the effective refractive index of the active layer, or a ridge waveguide may be disposed near a different type of ridge waveguide to perform a mode conversion. However, the gradual decrease of the thickness of a clad layer, and the configuration of different types of ridge waveguides complicate manufacturing processes, thus decreasing productivity.